U.S. patent number 7,337,875 [Application Number 10/878,806] was granted by the patent office on 2008-03-04 for high admittance acoustic liner.
This patent grant is currently assigned to United Technologies Corporation. Invention is credited to Christopher D. Jones, William P. Patrick, William Proscia.
United States Patent |
7,337,875 |
Proscia , et al. |
March 4, 2008 |
High admittance acoustic liner
Abstract
A cooled acoustic liner useful in a fluid handling duct includes
a resonator chamber 52 with a neck 56, a face sheet 86, and a
coolant plenum 80 residing between the face sheet and the chamber.
Coolant bypasses the resonator chamber, rather than flowing through
it, resulting in better acoustic admittance than in liners in which
coolant flows through the resonator chamber and neck. In one
embodiment, the liner also includes a graze shield 88. Openings 40,
38 penetrate both the face sheet and the shield to establish a
relatively low face sheet porosity and a relatively high shield
porosity. The shielded embodiment of the invention helps prevent a
loss of acoustic admittance due to fluid grazing past the liner.
Another embodiment that is not necessarily cooled, includes the
resonator chamber, low porosity face sheet and high porosity
shield, but no coolant plenum for bypassing coolant around the
resonator chamber. An associated method of retrofitting an acoustic
treatment into a fluid handling module includes installing openings
in the module and mounting a resonator box 44 on the module so that
the inlets to the resonator necks register with the installed
openings.
Inventors: |
Proscia; William (Marlborough,
CT), Jones; Christopher D. (Wethersfield, CT), Patrick;
William P. (Glastonbury, CT) |
Assignee: |
United Technologies Corporation
(Hartford, CT)
|
Family
ID: |
35124407 |
Appl.
No.: |
10/878,806 |
Filed: |
June 28, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050284690 A1 |
Dec 29, 2005 |
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Current U.S.
Class: |
181/214; 181/213;
181/292; 181/290; 181/210 |
Current CPC
Class: |
F02K
1/827 (20130101); G10K 11/172 (20130101); F02C
7/24 (20130101); F02C 7/045 (20130101); Y02T
50/671 (20130101); F05D 2260/96 (20130101); B64D
2033/0206 (20130101); Y02T 50/60 (20130101) |
Current International
Class: |
B64D
33/02 (20060101) |
Field of
Search: |
;181/214,210,213,290,292,293,294,296,229,230,231,295,224,291
;244/1N ;428/116,117,118 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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401226907 |
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Sep 1989 |
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JP |
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402071300 |
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Mar 1990 |
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JP |
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Primary Examiner: Donovan; Lincoln
Assistant Examiner: Phillips; Forrest
Government Interests
STATEMENT OF GOVERNMENT INTEREST
This invention was made under U.S. Government Contract
F33657-99-D-2051-0008. The Government has certain rights in the
invention.
Claims
We claim:
1. A gas turbine engine acoustic liner for attenuating noise
generated by working fluid flowing through said gas turbine engine
at a noise source, comprising: a resonator including a chamber
having an opening; a face sheet between the chamber and the working
fluid, said chamber opening registering with a corresponding
opening in said face sheet; and a coolant plenum disposed between
the face sheet and the chamber, such that fluid communication
between said plenum and said chamber opening is blocked to prevent
compromising the acoustic admittance of said chamber.
2. The liner of claim 1 comprising: a shield between the face sheet
and the working fluid; resonator openings penetrating the face
sheet and establishing a face sheet porosity; shield openings
penetrating the shield and establishing a shield porosity greater
than the face sheet porosity.
3. The liner of claim 2 including a distribution chamber between
the face sheet and the shield.
4. The liner of claim 2 wherein the ratio of the shield porosity to
the face sheet porosity is about 10:1.
5. The liner of claim 4 wherein the shield porosity is about
30%.
6. The liner of claim 1 or 2 including coolant passages perforating
the face sheet.
7. The liner of claim 1 or 2 including a neck that extends into the
interior of the chamber.
8. The liner of claim 7 wherein the neck comprises a tube extending
into the interior of the chamber.
9. The liner of claim 7 wherein the neck is folded.
10. The liner of claim 9 wherein the folded neck comprises a tube
projecting from the resonator opening into the chamber and an
opposing rim that circumscribes the tube to define an outlet.
11. The liner of claim 2 wherein the shield is part of a duct and
the face sheet is part of a strap mounted on the duct.
12. The liner of claim 11 wherein the strap is movable relative to
the duct.
13. A resonator box for a modulated gas turbine engine cooling
module, comprising: a floor; walls extending from the floor and
cooperating with the floor and with each other to define an array
of chambers; resonator necks, each having an inlet and comprised of
tubes projecting into the chambers; a removable cap having raised
rims opposing the tubes so that the rims circumscribe the tubes to
define outlets wherein the box is retrofittable onto an adjustable
cooling air modulation strap in the exhaust nozzle of said gas
turbine engine.
14. An acoustic liner for attentating noise generated at a noise
source, comprising: a resonator including a chamber; a face sheet
between the chamber and the noise source, the lace sheet having a
face sheet porosity; and a shield between the face sheet and the
noise source, the shield having a porosity greater than the face
sheet porosity.
15. The acoustic liner of claim 14 including a distribution chamber
between the face sheet and the shield.
16. A method of retrofitting a fluid handling module to improve its
noise signature, the module including a rotatable cooling air
modulation strap, the method comprising: providing a resonator box
having resonator inlets; installing resonator openings in the
strap; and mounting the box on the strap so that the inlets
register with the resonator openings.
Description
TECHNICAL FIELD
This invention relates to acoustic liners and particularly to a
liner having a high acoustic admittance for achieving superior
noise attenuation.
BACKGROUND OF THE INVENTION
Acoustic liners are used in fluid handling ducts to attenuate
undesirable noise associated with a stream of fluid flowing through
the duct. Examples of such ducts include the inlet and exhaust
system ducts of gas turbine engines. A typical acoustic liner
includes a back sheet, a face sheet spaced from the back sheet, and
a series of walls that extend between the face sheet and back sheet
to define an array of chambers. A set of holes or necks, usually
one per chamber, penetrates the face sheet to establish
communication between the chamber and the fluid stream. Each
chamber and its associated neck is a Helmholtz resonator tuned
(i.e. designed) to attenuate a narrow bandwidth of noise
frequencies depending on the area and length of the neck, the
volume of the chamber, and the local speed of sound. The liner is
positioned along the duct wall with the face sheet extending
approximately parallel to the direction of fluid flow through the
duct.
During operation, the array of resonators attenuates noise
attributable to pressure disturbances in the fluid stream. The
effectiveness of a resonator in attenuating noise at its design
frequency range depends on its ability to admit the disturbance
into the chamber, a property referred to as acoustic admittance.
Alternatively, the inability of a resonator to receive a
disturbance is referred to as acoustic impedance, a complex
quantity whose real component is known as resistance.
Despite the many merits of Helmholtz resonator acoustic liners,
various factors can degrade their acoustic admittance. For example,
when the liner is used to line a fluid handling duct, the flowing
fluid grazes past the inlets to the resonator necks and, in doing
so, reduces the acoustic admittance of the resonators. This occurs
because the grazing fluid produces a region of fluid recirculation
inside the neck, which reduces the effective area of the neck,
thereby decreasing the acoustic admittance.
Another factor that can degrade acoustic admittance is the flow of
coolant through the resonator necks. The use of coolant is often
necessary when the duct carries a stream of high temperature fluid,
such as the combustion products that flow through a turbine engine
exhaust system duct. Cooling is normally accomplished by
introducing the coolant (usually relatively cool air) into the
resonator chambers through the chamber walls or through the liner
back sheet. The coolant then flows out of the chambers by way of
the resonator necks. The coolant flowing through the necks reduces
the acoustic admittance of the resonator. The loss of acoustic
admittance becomes more severe with increasing coolant Mach
number.
Specific applications for acoustic liners and specific constraints
imposed on their design can present additional challenges. For
example, if the need to cool an acoustic liner was not anticipated
during the early stages of product design, it can be challenging to
retrofit a cooled liner into the product without adversely
affecting other attributes of the product. And, irrespective of
whether the need for an acoustic liner was appreciated early in
product design, the severe space constraints faced by the designer
of a gas turbine engines make it inherently difficult to design a
liner that attenuates low frequency noise. This is because the
design frequency of a Helmholtz is inversely proportional to
chamber volume. Hence, large volumes (large amounts of space) are
required to attenuate low frequency noise.
SUMMARY OF THE INVENTION
It is, therefore, an object of the invention to provide an acoustic
liner that exhibits high acoustic admittance despite the presence
of cooling flow.
It is another object of the invention to provide an acoustic liner
that exhibits high acoustic admittance despite the presence of
grazing flow past the liner.
It is another object to enable an acoustic treatment to be retrofit
into a product in which the need for the treatment was not
originally anticipated.
It is another object to provide an acoustic liner whose high
admittance helps compensate for the difficulty of attenuating low
frequency noise in applications where severe constraints are
imposed on the volume of the Helmholtz resonator chamber.
According to one embodiment of the invention, an acoustic liner
includes a resonator chamber and a face sheet. A coolant plenum
resides between the face sheet and the chamber. Coolant bypasses
the resonator chamber, rather than flowing through it, resulting in
improved acoustic admittance.
In another embodiment, the liner also includes a shield between the
face sheet and the noise source. Openings penetrate both the face
sheet and the shield to establish a relatively low face sheet
porosity and a relatively high shield porosity. The shielded
embodiment of the invention helps prevent a reduction of acoustic
admittance due to fluid grazing past the liner.
Another embodiment, one that is not necessarily cooled, includes a
resonator chamber, a relatively low porosity face sheet and a
relatively high porosity shield, but no coolant plenum for
bypassing coolant around the resonator chamber.
One form of the invention employs an internally partitioned
resonator box with a removeable cap. Tubes projecting from the
floor of the box cooperate with opposing rims projecting from the
cap to define a set of folded resonator necks. The box offers a
convenient way to retrofit an acoustic treatment into an untreated
fluid handling duct.
An associated method of retrofitting a duct or similar fluid
handling module includes installing openings in the module and
mounting a resonator box on the module so that the inlets to the
resonator necks register with the installed openings.
These and other embodiments and features of the invention will now
be described in more detail in the following description of the
best mode for carrying out the invention and the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross sectional side elevation view of the aft end of
an afterburning gas turbine engine with the engine casing partially
broken away to expose internal components of engine including a
modulated exhaust cooling (MEC) module.
FIG. 2A is an exploded perspective view showing the MEC module of
FIG. 1 including an MEC duct, an MEC strap and a resonator box with
an array of resonators for a cooled acoustic liner.
FIG. 2B is a cross sectional side elevation view showing an
acoustic liner employing the resonator array and MEC strap but not
the MEC duct of FIG. 2A.
FIG. 2C is a cross sectional side elevation view showing an
acoustic liner employing the resonator array, MEC strap and MEC
duct of FIG. 2A.
FIGS. 3A and 3B are a perspective view of a resonator box and a
cross sectional side elevation view of an alternate embodiment of
the inventive acoustic liner.
FIGS. 4A and 4B are a perspective view of a resonator box and a
cross sectional side elevation view of another alternate embodiment
of the inventive acoustic liner.
FIGS. 5A and 5B are a perspective view of a resonator box and a
cross sectional side elevation view of an acoustic liner that does
not necessarily require cooling.
FIGS. 6A and 6B are an alternative to the embodiment of FIGS. 5A
and 5B.
BEST MODE FOR CARRYING OUT THE INVENTION
Referring to FIGS. 1 and 2A, an afterburning, variable cycle gas
turbine engine for a high performance aircraft includes an outer
case 10 and an internal centerbody 12 circumscribing an engine axis
14. The engine internal components also include an array of
circumferentially distributed turbine exhaust vanes 16, an
afterburner flameholder 18 integral with each vane 16, a modulated
engine cooling (MEC) module 20, and an afterburner duct 22. A
working medium flowpath 24 extends axially along the length of the
engine. During engine operation, a fluid stream F comprising hot
combustion gases flows through the vane array 16, the MEC module
20, and the afterburner duct 22. When the aircraft pilot selects
afterburning operation, the engine fuel system introduces fuel into
the combustion gases in the vicinity of the flameholder. The
combustion gases ignite the fuel to produce additional thrust.
A pre-existing MEC module 20 includes an MEC duct 28 and an MEC
strap 30 encircling the duct. The strap 30 is rotatably mounted on
the duct so that actuators, not illustrated, can rotate the strap
relative to the duct about axis 14. Various openings penetrate
through the MEC strap to define circumferentially alternating
regions S.sub.HP, S.sub.LP of high and low strap permeability.
Specifically, large windows 32 penetrate the strap in region
S.sub.HP while numerous coolant passages 34 penetrate the strap in
region S.sub.LP. The openings 40 shown penetrating the strap in
regions S.sub.LP are not present in the pre-existing MEC strap and
hence are not related to its permeability as just described.
Various openings also penetrate through the MEC duct 28 to define
circumferentially alternating regions D.sub.LP, D.sub.HP of low and
high duct permeability. Specifically, numerous coolant passages 36
(visible through the windows 32) penetrate the duct in region
D.sub.LP while a smaller quantity of larger openings 38 penetrate
the duct in region D.sub.HP.
During engine operation, a control system rotates the MEC strap 30
relative to the MEC duct 28 to align the high and low permeability
regions of the strap with the high and low permeability regions
respectively of the duct or to align the high and low permeability
regions of the strap with the low and high permeability regions
respectively of the duct. The alignment or misalignment of the
regions of high and low permeability regulates the admission of
coolant according to engine requirements and also adjusts the
engine bypass ratio to alter the thermodynamic cycle of the
engine.
At certain engine operating conditions, use of the afterburner
results in a highly undesirable noise signature that originates in
the engine flowpath 24. These operating conditions correspond to
those where the MEC strap regions S.sub.LP are aligned with MEC
duct regions D.sub.HP as seen in FIG. 2A. The undesirable noise
signature may be attenuated by employing an acoustic liner
according to an embodiment of the invention described below.
Referring now to FIGS. 2A through 2C, a noise attenuating resonator
box 44 includes a floor 46, exterior wall 48 extending from the
floor and interior walls 50 also extending from the floor. The
walls 48, 50 cooperate with the floor and with each other to define
an array of resonator chambers 52. Openings 54 penetrate the floor,
one opening per chamber. An internal tube 60 projects from each
opening 54 into the interior of the chamber. An external tube 62
projects away from the chamber. The resonator box also includes a
removeable cap 66 having rims 68. Screws 70, secure the cap in
place. Other means of securing the cap, such as brazing, may also
be used. A seal, not illustrated, resides between the cap and the
walls 48, 50 to prevent fluid leakage into or out of the box and
between the individual chambers 52. When the cap is installed, each
rim opposes and circumscribes an internal tube.
The resonator box also includes a pair of circumferentially
extending mounting lugs 74. The box is securable to the MEC strap
30 by pins 76 that project through holes in the lugs and are brazed
to rails 78 on the strap. The external tubes 62 serve as standoffs
to define a coolant plenum 80 between the floor 46 and the MEC
strap 30. The end of each tube 62 is brazed to the MEC strap to
provide a fluid tight seal.
The openings 40, which are not present in pre-existing MEC straps,
are installed in the strap so that when the resonator box is
mounted on the strap as just described, the openings 40 register
with the external tubes 62. Openings 40 are therefore referred to
as resonator openings. Each resonator opening has an inlet 42. In
the embodiments of FIGS. 2B and 2C, each opening 40, external tube
62, internal tube 60 and rim 68 define a folded resonator neck 56
having an outlet 72. The resonator neck establishes communication
between the chambers 52 and their local environment. Other neck
configurations, some of which are described later, may also be
used.
Disregarding for the moment the MEC duct 28, the above described
assembly defines an acoustic liner, as best seen in FIG. 2B, for
attenuating noise generated at a noise source in the engine
flowpath 24. The liner comprises a resonator defined by at least
one chamber 52 and by MEC strap region S.sub.LP which serves as a
acoustic liner face sheet 86 residing between the chamber and the
noise source. During engine operation, the chamber and its
associated neck 56 comprise a Helmholtz resonator that attenuates
noise produced at the noise source. In addition, coolant from
plenum 80 flows through passages 34 to cool the face sheet 86.
Because this coolant completely bypasses the neck 56 rather than
flowing through it, the coolant flow does not adversely affect the
acoustic admittance of the acoustic liner. Hence the liner is
better able to attenuate noise in its design frequency band than
would be the case if coolant discharged conventionally through the
neck 56. Alternatively, some fraction of the coolant could pass
through the chambers and necks, with the balance of the coolant
bypassing the resonators as just described. Although the coolant
that flows through the neck will degrade the acoustic admittance,
the loss of acoustic performance will be less noticeable than if
none of the coolant bypassed the resonators.
Referring now to FIGS. 2A and 2C, an alternative embodiment is
presented in the context of a turbine engine that includes both the
MEC strap 30 (acoustic liner face sheet 86) and the MEC duct 28. As
seen best in FIG. 2C, the MEC duct 28 and MEC strap 30 are locally
radially offset from each other to define a distribution chamber
82. The MEC duct, as will be explained, serves as a graze shield.
The duct regions D.sub.HP act as duct regions D.sub.H of relatively
high acoustic porosity due to the previously described openings 38.
The porosity of region D.sub.H is the aggregate area of openings 38
divided by the area of region D.sub.H, which is the product of
dimensions L.sub.1 and L.sub.2. The strap regions S.sub.LP, which
are circumferentially aligned with duct regions D.sub.H, act as
relatively low acoustic porosity strap regions S.sub.L. These low
acoustic porosity strap regions S.sub.L are the same regions
previously described as low permeability strap regions S.sub.LP in
the context of supplying coolant and altering engine bypass ratio.
However in the context of acoustic performance, the low acoustic
porosity of regions S.sub.L results from the resonator openings 40
penetrating the strap 30 (i.e the acoustic liner face sheet 86) not
from the passages 34 which are meaningful only in the context of
cooling and thermodynamic cycle. Hence, the acoustic porosity of
the strap 30 (face sheet 86) is the aggregate inlet area of
resonator openings 40 divided by the area of region S.sub.L, which
is approximately L.sub.1 times L.sub.2. In other words, the
collective area of the passages 34 does not contribute to the
acoustic porosity of the face sheet. The porosity of the duct
regions D.sub.H exceeds the porosity of the strap regions
S.sub.L.
During operation, the acoustic damping afforded by the liner is
required, and is achieved, when the MEC strap is positioned so that
strap region S.sub.L is circumferentially aligned with MEC duct
region D.sub.H. Coolant flows through passages 34, into
distribution chamber 82, and through the openings 38 which exhaust
into the flowpath 24. Because the coolant bypasses the Helmholtz
resonators rather than flowing through the resonator chambers and
necks, its presence does not degrade the acoustic admittance of the
liner. The effect of coolant discharging through openings 38 is a
low velocity discharge that does not appreciably degrade the
acoustic admittance of the liner. The MEC duct regions D.sub.H also
help preserve the liner's acoustic admittance by acting as a graze
shield between openings 40 and the fluid stream F.
Accordingly, an alternate embodiment of the inventive acoustic
liner also includes a shield 88 between the face sheet 86 and the
noise source in flowpath 24. The shield is offset from the face
sheet to define a distribution chamber 82. The shield has an
acoustic porosity attributable to the shield openings 38. The
acoustic porosity of the shield is the collective area of openings
38 divided by the total area of region D.sub.H, which is
approximately L.sub.1 times L.sub.2. The porosity of the shield
exceeds the porosity of the face sheet. The presence of the higher
porosity shield limits the loss of acoustic admittance that would
normally occur as a result of the combustion gases grazing axially
past the face sheet 86. This occurs for two reasons. First, because
the face sheet is not exposed to the grazing flow, the region of
fluid recirculation produced by the grazing flow occurs at the
graze shield openings 38 rather than at the resonator openings 40.
Second, due to the higher porosity of the graze shield relative to
that of the face sheet, the acoustic velocity fluctuations in the
shield are smaller as a result of conservation of mass. Therefore,
the acoustic resistance is controlled by the face sheet resistance
and is not limited by the graze shield resistance.
In one specific embodiment of the invention of FIGS. 2A and 2C,
each resonator opening 40 has a diameter of about 0.170 inches
(approximately 4.3 mm) resulting in a face sheet porosity of about
3%. Each shield opening 38 has a diameter of about 0.148 inches
(approximately 3.8 mm). Although the shield openings are slightly
smaller in diameter than the resonator openings, they are more
numerous, resulting in a shield porosity of about 30%. Hence, the
ratio of shield porosity to face sheet porosity is about 10:1.
FIGS. 3A and 3B show an acoustic liner with an extended neck 56,
rather than the folded neck of FIGS. 2A through 2C. FIGS. 4A and 4B
show an embodiment with a simple neck, neither extended nor folded.
The designer can use these and other neck variations to tune the
liner to a desired frequency band without affecting its external
dimensions.
Referring again to FIG. 2A, the resonator box offers a convenient
way to retrofit a duct with an acoustic treatment whose
desirability may not have been appreciated during the early stages
of product design and development. In a pre-existing MEC module the
strap includes regions S.sub.HP with windows 32, and regions
S.sub.LP with passages 34, but without resonator openings 40. The
MEC duct includes regions D.sub.LP with passages 36 and regions
D.sub.HP with openings 38. Noise attenuation is desired when the
regions S.sub.LP are circumferentially aligned with regions
D.sub.HP as depicted in FIG. 2A. To provide the desired noise
attenuation it is necessary only to install the resonator openings
40 in strap regions S.sub.LP, and mount a resonator box in each
region S.sub.LP so that the necks 56 register with the resonator
openings 40.
FIGS. 5A and 5B, show an additional embodiment that is not
necessarily cooled. In this embodiment of the acoustic liner, the
external tubes 62 of FIGS. 2B, 3B and 4B are absent so that the
floor 46 of the resonator box contacts the face sheet 86. From an
acoustic standpoint there is no distinction between the floor 46
and the face sheet, and they may be constructed as a single unit.
The face sheet resides between resonator chambers 52 and the noise
source in flowpath 24. The face sheet has a face sheet porosity
attributable to the resonator openings 40. A graze shield 88
resides between the face sheet and the noise source and is spaced
from the face sheet to define a distribution chamber 82. The shield
has a porosity attributable to shield openings 38. The shield
porosity exceeds the face sheet porosity. As a result, the acoustic
admittance of the liner is not degraded by the grazing action of
the fluid stream F as it flows past the liner.
FIGS. 6A and 6B, show yet another embodiment that is not
necessarily cooled. In this embodiment, the shield 88 contacts the
face sheet 86 so that there is no distribution chamber such as
chamber 82 of FIG. 5B. In addition, the quantity of shield openings
38 equals the quantity of resonator openings 40 and each opening
38, is aligned with an opening 40. Moreover, the diameters of the
shield openings are larger than those of the resonator openings so
that the shield porosity exceeds the face sheet porosity. In the
illustrated embodiment, the diameter ratio is about 3:1, which
results in a shield porosity approximately 10 times that of the
face sheet porosity. As with the other shielded embodiments, the
shield reduces or eliminates fluid recirculation in resonator
openings 40. In addition, the high porosity of the shield relative
to that of the face sheet reduces the acoustic resistance of
openings 38 so that the grazing action of the fluid stream F does
not significantly degrade the acoustic admittance of the liner.
Although the embodiments of FIGS. 5 and 6 are not necessarily
cooled, cooling can be accomplished, if desired, by introducing
coolant into the chambers 52 and exhausting it through openings 38.
In such a liner, coolant inlet passages (not illustrated) would
penetrate the resonator exterior wall 48 and/or cap 66 to meter
coolant into the chambers 52. The coolant would then flow through
the resonator chambers 52, and through the illustrated resonator
openings 40 and shield openings 38.
The invention has been described in the context of a turbine engine
having an MEC strap, an MEC duct and a distinct resonator box.
However this is merely one specific arrangement that takes
advantage of pre-existing openings 38 in a turbine MEC duct and,
with nothing more than a resonator box and the installation of
openings 40 in the MEC strap, introduces an acoustic treatment into
a previously untreated duct. Moreover, the resulting acoustic
treatment has a high acoustic admittance for superior noise
attenuation. In general, the disclosed liner is a generic acoustic
liner applicable to a wide range of products.
Acoustic liners constructed according to the invention may comprise
numerous resonators all tuned to the same frequency. Alternatively,
the resonators may be tuned to different frequencies to achieve
broadband noise attenuation. Moreover, the liner may be configured
so that a cavity communicates with its environment by way of
multiple necks.
Although this invention has been shown and described with reference
to detailed embodiments thereof, it will be understood by those
skilled in the art that various changes in form and detail may be
made without departing from the invention as set forth in the
accompanying claims.
* * * * *